Each park registers a background noise level below 20 decibels, a silence so deep the researchers believe that the areas have remained sonically untouched since European colonization of the Americas in the tenth century, Science reported.

For comparison, most cities range from 50 to 60 decibels.

This data comes from 1.5 million hours of acoustic monitoring across the entire country. The National Park Service recorded data everywhere in the Lower 48, from the busiest cities to remote parks, and, where they couldn’t record data, researchers used a computer algorithm to predict noise pollution based on levels of air and street traffic.

A map of the data shows that the entire eastern half of the country is drastically louder than the West. While the east has its patches of quiet, most of it a haze of mid-level background noise that swells to a roar near big cities. The West, however, is almost entirely quiet, with the distinct exceptions of the major cities: Los Angeles, San Diego, San Francisco, Phoenix, Portland, Ore. and Seattle.

The National Park Service says that they will use the data to study how manmade noise affects wildlife across the country. Animals such as bats and owls have a much more acute sense of hearing – up to 20 decibels more sensitive than humans. Noise from humans can drown out sound made by the insects and rodents that theses animals rely on to survive.

In “protecting the natural soundscape” of even one square inch of wilderness, the organization hopes that, because sound travels long distances in natural areas, miles of surrounding forest will benefit from the decrease in noise pollution.

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